The long-term goal of our laboratory is to better understand the age-based differences in host immunity that affect acute and chronic pulmonary disease outcomes so we can improve therapeutic and vaccine strategies for infant and elderly populations. Our current research focus is respiratory syncytial virus (RSV), an infection that causes severe disease in the very old and very young, has no available vaccine and no effective therapy. We have developed an infant mouse model of RSV infection to improve our understanding of infant versus adult host immune responses to RSV infection. It is known is that the accumulation of cellular debris and airway occlusion is a consequence of severe RSV disease and alveolar macrophages (AMs), which make up >90% of airway cells, are responsible for clearing cell debris. However, little is known about how immature infant AMs contribute to severe disease or their potential for immunomodulation to mitigate RSV pathology. The goal of the proposed work is to determine the extent to which infant AMs prolong RSV replication and impair removal of apoptotic cells during infection and if immune modulation with inhaled IFN? mitigates these effects. We have shown that treatment with inhaled IFN? increases AM activation, expedites RSV clearance, and reduces the number apoptotic cells and mucus in infant BALB/c mice. We hypothesize that infant AMs contribute to the accumulation of apoptotic cellular debris and prolong RSV replication due to an immature lung environment which can be mitigated with inhaled IFN?. To test our hypothesis we will complete two Specific Aims. The first Aim is to determine the mechanisms by which inhaled IFN? enhances the clearance of RSV, apoptotic cells (ACs), and mucus and protects against RSV-mediated airway hyperresponsiveness (AHR) in neonatal mice. Results of this aim are expected to show that mice with depleted AMs will have reduced M1 and M2c AM function which is required for clearance of ACs, delayed RSV clearance, and increased airway resistance and that IFN? acts on AMs to resolve these effects. Specific Aim 2 will define the signals that regulate the efferocytosis of RSV-laden A549 cells by human cord and adult blood monocyte-derived macrophages with or without IFN priming. We expect IL-10/M-CSF polarized M2c macrophages, regardless of source, will increase AC clearance more than TGF-polarized macrophages during RSV infection, but in the absence of T-cell recruitment, inhaled IFN? will mitigate RSV-mediated impaired efferocytosis.